Fluorescent dyes are widely used as tracers for localization of biological structures by fluorescence microscopy, for quantification of analytes by fluorescence immunoassay, for flow cytometric analysis of cells, for measurement of physiological state of cells and other applications (Kanaoka, Angew. Chem. Intl. Ed. Engl. 16: 137 (1977); Hemmila, Clin. Chem. 31: 359 (1985)). Among the advantages of fluorescent agent over other types of absorption dyes include the detectability of emission at a wavelength distinct from the excitation, the orders of magnitude greater detectability of fluorescence emission over light absorption, the generally low level of fluorescence background in most biological samples and the measurable intrinsic spectral properties of fluorescence polarization (Jolley et al., Clin. Chem. 27: 1190 (1981)), lifetime (U.S. Pat. No. 4,374,120) and excited state energy transfer (U.S. Pat. Nos. 3,996,345; and 4,542,104).
Fluorescent agents are now widely used to determine physiological functions in patients during routine checkups or diagnostic procedures, to monitor the exposure of workers and others to potentially harmful chemicals such as toxic or carcinogenic pesticides or inorganic materials in the atmosphere, soil, or drinking water, in determining the effectiveness of pharmaceuticals on disease states or conditions, in screening new compounds for biological activity as either promoters or inhibitors of particular enzymes, in monitoring gene and transgene expression, and in performing immunological and other laboratory assays such as enzyme-linked immunosorbent assays (ELISAs) and Western blots.
Optical methods of detection, such as fluorescence emission, UV absorptivity, and colorimetry are convenient and highly effective for detecting, monitoring, and measuring fluorescent agents, since methods such as these can generate either qualitative or quantitative information and detection can be achieved either by direct visual observation or by instrumentation.
For many applications that utilize fluorescent dyes as tracers, it is necessary to chemically react the dye with a biologically active ligand such as a cell, tissue, protein, antibody, enzyme, drug, hormone, nucleotide, nucleic acid, polysaccharide, lipid or other biomolecule to make a fluorescent ligand analog or to react the dye with natural or synthetic polymers. With these synthetic probes, the biomolecule frequently confers a specificity for a biochemical interaction that is under investigation and the fluorescent dye provides the method for detection and/or quantification of the interaction. Thus, useful dyes are based on a versatile fluorescent nucleus that allows the preparation of reactive derivatives of several different types that exhibit reactivity toward a variety of chemically reactive sites.
There is a recognized need for suitable fluorophores, particularly reactive fluorophores, for applications in multi-color, multiplexed applications, such as microscopy, flow cytometry, immunoassays, and nucleic acid sequencing. Most of the dyes proposed for these applications have longer wavelength emission than fluorescein. Since fluorescein has essentially no fluorescence below 490 nm, a well-designed assay system utilizing fluorescein and a second fluorophore that has a strong emission below this wavelength, would be useful for multiplex assays.
A number of dyes have been proposed in the literature that can be excited and detected at wavelengths less than 500 nm. Several chemically reactive fluorophores that can be excited in the near ultraviolet and short wavelength visible region of the spectrum have been described. Exemplary tracers are derived from naphthalene derivatives such as 5-dimethylaminonaphthalene-1-sulfonic acid (Dansyl) (Weber, Biochem. J. 41: 145 (1952); Rinderknecht, Experientin 16: 430 (1960)) 3-(isothiocyanato)naphthalene-1,5-disulfonic acid (Braunitzer et al., Hoppe-Seyler's Z. Physiol. Chem. 352: 1730 (1971)), and N-(4-methylphenyl)-5-isothiocyanato-1,8-naphthalimide (Khalaf & Rimpler, Hoppe-Seyler's Z. Physiol. Chem. 358: 505 (1977)); pyrene derivatives such as pyrene-1-butyric acid (PBA) (Malencik & Anderson, Biochemistry 11: 3022 (1972)), 8-isothiocyanatopyrene-1,3,6-trisulfonic acid (IPTS) (Braunitzer, et al., Hoppe-Seyler's Z. Physiol. Chem. 354: 1536 (1973)) and 8-hydroxypyrene-1,3,6-trisulfonyl chloride (Wolfbeis, Fresenies Z. Anal. Chem. 320: 271 (1985)); stilbene derivatives such as 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS) (Cornelisse & Ploem, J. Histochem. Cytochem. 24: 72 (1976)); and coumarin dyes such as 3-carboxy-7-hydroxycoumarin (Baker & Collis, J. Chem. Soc. 1949, S 12 (1949)), 7-hydroxy-4-methylumbelliferone-3-acetic acid (4-MUA) (Khalfan, et al., Biochem. J. 209: 265 (1983)), 7-amino-4-methylcoumarin-3-acetic acid (AMCA) (Khalfan, et al., Histochem. J. 18: 497 (1986)) and 7-diethylaminocoumarin (Staines et al., J. Histochem. Cytochem. 36: 145 (1988))
The existing, reactive, blue-fluorescent fluorophores generally have a weak absorbtivity (extinction coefficients of less than 20,000 cm−1 M−1 at their absorbance maxima), relatively low quantum yields, and/or or not particularly well solubilized in aqueous environments. Such properties are less than ideal for a fluorophore of interest for biological applications.
For example, the large Stokes' shifts and wide emission band-widths of many art-recognized dyes result in significant residual fluorescence background from the ultraviolet excited dyes at wavelengths typically used for detection of fluorescein emission (typically 515 to 525 nm). With the exception of 4-MUA, the potential alternative dyes are not optimally suited for excitation with the strongest emission lines of the most commonly available sources, such as the 365 nm line of the mercury arc lamp. Moreover, fluorescence of many of the art-recognized dyes is frequently quenched in aqueous solution, resulting in low quantum yields. The lower quantum yield decreases the detection sensitivity or requires use of disproportionately larger quantities of the less fluorescent dye.
Despite their acceptance as fluorescent tracers, coumarin derivatives have deficiencies that preclude or make more difficult some useful applications. The reactive derivatives of aminocoumarins are quite lipophilic and insoluble in aqueous preparations. Additionally, hydroxycoumarin derivatives typically exhibit pKa values near or above 7.0 and show a pH dependent absorption spectrum that displays a decreased fluorescence in the physiological pH range.
While having other desirable properties of high absorbance and high water solubility, SITS has a very low fluorescence yield in water and is photolytically isomerized to the non-fluorescent cis isomer. Despite the commercial availability of SITS for many years, use of SITS to form fluorescent conjugates has not been widely adopted due to the low fluorescence yield. SITS and related stilbene derivatives also have been found to have an inhibitory effect on anion transport systems in red blood cells (Barzilay, et al., Membrane Biochemistry 2: 227 (1979)) and on microsomal glucose-6-phosphatase (Speth & Schulze, Eur. J. Biochem. 174: 111 (1988)). Another drawback of SITS is the short wavelength absorption maximum (<350 nm). Ultraviolet excitation of SITS can result in cell injury and death in applications where fluorescence measurements are performed on living cells. Autofluorescence of proteins, nucleic acids and other biomolecules present in cells is also increased with shorter wavelength excitation.
Furthermore, the emission spectra of stilbene, napthalene and coumarin derivatives have a very broad long wavelength component which greatly increases the fluorescence background at wavelengths used for detection of other dyes such as fluorescein, Lucifer Yellow and tetramethylrhodamine in applications such as DNA sequencing, developmental tracers and flow cytometry that require detection of multiple dyes and dye conjugates.
With the exception of SITS and the isothiocyanate of pyrenetrisulfonic acid, the solubility in aqueous solution of the commonly used reactive forms of these dyes is very low, necessitating use of organic solvent co-mixtures in forming dye conjugates with most biopolymers. Reactive dye derivatives such as succinimidyl pyrene-1-butyrate and the succinimidyl ester of AMCA are quite lipophilic and insoluble in the aqueous solutions required for fluorescent labelling of proteins, polysaccharides and other biomolecules.
In certain applications, it is desired to utilize a rigid fluorophore to assess the local environment proximate the fluorophore. If the motional correlation times of the fluorophore are long relative to its fluorescence lifetime, the environment in which the fluorophore is located can be probed by a variety of energy transfer experiments, including fluorescence resonance energy transfer, a method that can be used to determine the distance between an immobilized fluorophore and a resonant group such as tryptophan. A rigid fluorophore will generally exhibit motional correlation times that are enhanced relative to more flexible or sterically undemanding fluorophores, thereby enhancing the energy transfer between the fluorophore and another group.
Haugland et al. disclosed reactive tri-sulfonic acid pyrene fluorophores, however, the linker arms incorporated into these agents do not incorporate identical rigid structures, nor do they include the rigid groups as near to the fluorophore, as the compounds of the invention (U.S. Pat. No. 5,132,432). For example, Haugland et al. discloses compounds that include a ring structure formed with nitrogen atom (NRR′) in the —OCH2CO2N(CH2)vNRR′, in which v is 2 or 5. In contrast, exemplary compounds of the invention include a cyclic structure formed between the nitrogen and R′ and R″ moieties of a linker arm of formula —OCH2CO2NR′R″.
In view of the above, a fluorophore having a reactive group attached to the fluorescent nucleus of the fluorophore via a rigid linker arm, which is also water soluble, and highly fluorescent within a narrow wavelength range would be a highly desirable addition to the art-recognized array of reactive fluorophores. The present invention provides such fluorescent agents, conjugates incorporating the agents and methods of using the agents and their conjugates.